This invention relates to a process and a device for depositing a thin layer using a plasma-CVD technique on a conductive substrate where the substrate itself is used as an electrode for an electrical current. Of particular interest, an insulating substrate can be used that is first made conductive by the deposition of a thin conductive layer. Advantageously, the thin conductive layer will remain intact when the final product is obtained.
The invention also relates to a substrate covered by thin layers including at least one metal layer, preferably silver, on which a layer such as an organosilicon layer is deposited according to the process of the invention.
It is known to protect the surfaces of various materials by depositing on the surface a thin layer, such as one consisting of an organic compound obtained by the chemical reaction of a monomer under a low pressure plasma. Thus, French patent application FR 85 18673 and corresponding U.S. patent applications Nos. 515,539 and 306,960 describe an organosilicon film obtained by the introduction, under primary vacuum, of an organosilicon monomer gas into an electric discharge created by the capacitive coupling between two electrodes. One electrode is grounded and supports the substrate to be coated, the other is located a small distance above the substrate and is connected to a power generator operating at frequencies below one megahertz. This process makes it possible to obtain thin layers which have many applications including, the protection of thin layers with silver bases deposited on a glass substrate.
In the above mentioned process, the substrate remains immobile during the deposition. In other known processes the plasma is contained with a magnetic field and the substrate is displaced relative to the electrodes and therefore relative to the plasma.
Such a process is cumbersome to use because it requires a mechanical drive system under vacuum which is difficult to achieve and maintain. Further, since the thin organosilicon layers are most often intended to protect the more fragile operational layers, such as silver, an additional chamber may be required to facilitate industrial operations. For example, on an industrial production line where the operational layers are deposited, it would be necessary to have a special chamber equipped with a mechanical drive system where the deposition of the final protective layer could be performed. This chamber would have to be similar to those where the first layers are deposited, aside from its mechanical requirements, and be placed before the output lock chamber (a chamber where the pressure is normally brought to atmospheric pressure) and after the other deposition chambers.
The requirement of a special chamber can be obviated by using the above mentioned process of EP 230 188 where the substrate remains immobile. Since this process makes it possible, under certain limits, to perform the deposition without relative displacement between the glass and the plasma, it can be performed, for example, during the substrate's stop in the input lock chamber or the output lock chamber of an industrial cathode-sputtering line.
However, this process also has its limits. It has been observed that the zone where the major portion of the polymer deposition is performed is that closest to the electrode located above the substrate. Additionally, the deposition is performed as quickly on the upper electrode as on the substrate to be treated. This has multiple negative consequences. First, the quantitative efficiency of the process is low since only a small proportion of the monomers introduced in the chamber are found in the polymer on the glass. Furthermore, the operation of the process itself is impeded by the preferred deposition on the upper electrode because the deposition, which is insulating, modifies the characteristics of the electrical field created under the electrode. This in turn creates a drift that must be corrected by adjusting the electrical parameters of the process. Additionally, a harmful mechanical effect can also be manifested when several glass sheets are treated successively. In this situation the deposition on the electrode increases in thickness as the successive sheets are treated; and the thickening layer becomes friable and eventually disintegrates. Chips then fall on the substrate where they disturb the deposition process. Further, the rate of deposition on the glass is not very high and the thickness of the final layer is greater at the center than it is opposite the edges of the electrode.